To save space, the elevators installed in tall buildings are generally designed for fast operation and heavy use. These elevators are required to have a high transport capacity. The number of starts in a year may amount to several hundred thousand. Fast elevators and elevators with a large travel height are generally implemented as traction sheave elevators. The hoisting ropes connecting the elevator car and the counterweight of a traction sheave elevator usually run over the traction sheave and at least one diverting pulley. The hoisting motor of the elevator imparts rotation to the traction sheave either directly or via a gear. The rotary motion of the traction sheave is converted into longitudinal motion of the ropes by means of the friction between the traction sheave and the ropes. Creating a large frictional effect between the traction sheave and the ropes promotes the usability of the elevator. A large frictional effect is achieved fairly easily by increasing the angle of contact between the ropes and the traction sheave. However, increasing the angle of contact often results in an increased number of deflections, causing wear of the ropes. The strain resulting from deflection is more severe in cases where the ropes are deflected in a direction opposite to that of the previous deflection. Such a deflection is termed reverse deflection. The deflections may take place in the plane of the traction sheave or in the planes of the diverting pulleys guiding the ropes. Moreover, the ropes may also be deflected in an oblique direction from the plane of rotation of the pulley as they enter onto the pulley or leave it. This deflection is referred to as skew traction angle. The grip of the ropes on the traction sheave can be increased by increasing the coefficient of friction between the rope and the traction sheave, or by shaping the rope grooves of the traction sheave so that the ropes will be compressed in the grooves. However, a disadvantage is that increasing the friction coefficient and compression of the rope through the shaping of the groove result in a reduction cf the service life of the ropes and the traction sheave, especially of the rope grooves of the sheave.-The abrasion can be reduced by increasing the size of and the distance between the traction sheave and the diverting pulleys, but such a solution would result in increased manufacturing costs. Also, the assembly formed by the elevator machine and its bed and the traction sheave and diverting pulleys would be so large that it would be difficult or impossible to house it in a conventional elevator machine room. Even with the present sizes of traction sheaves and diverting pulleys, it necessary to use lefthand and righthand types of machine and machine bed to enable all the required equipment to be fitted in machine rooms which are often very small.
In a previously known traction sheave elevator, presented by Finnish patent 56813, the ropes connecting the counterweight and the elevator car are deflected by a diverting pulley onto the traction sheave and run around it bending in the opposite direction, whereupon they run further back into the elevator shaft, possibly over another diverting pulley. The angle of contact of the ropes is 210.degree.-250.degree. and the skew traction angle of the ropes entering and those leaving the traction sheave is 1.degree. from the plane of rotation of the sheave to ensure that the ropes will not touch each other at the crossover point. The rope grip is further improved by undercutting the rope grooves of the traction sheave. In its time, the solution presented by Finnish patent 56813 allowed economies with respect to space as it made it possible to use a smaller traction sheave than before, which further permitted lighter machine structures. Finnish patent number 84051 presents a traction sheave elevator in which the skew traction angle resulting from ropes-running as in patent 56813 is influenced by tilting and turning the drive machine and its traction sheave so that the ropes meet the diverting pulley in the direction of its plane of rotation.
Finnish patent number 77207 presents a traction sheave elevator in which the ropes run similarly to the single-wrap traction sheave elevator of Finnish Patent 56813 except that the ropes run from the traction sheave to an additional diverting pulley and back to the traction sheave before being passed back into the shaft. The result is a so-called double-wrap elevator in which the contact angle may be 400.degree.-540.degree.. The large contact angle ensures a good frictional grip even if half-round rope grooves are used in the traction sheave.